I am pretty sure the answer to this is ED-E i was in a situation where he should have died but he didnt
It appears that companions(at least the ones I've worked with so far) have a part in the quests, Veronica gets you into the BoS secret hideout, ED-E i has a recorded message concerning an enclave base, in my game they are set essential, which I admit came as a pleasant surprize, but obviously this can make the game a little too easy, just stay back and let them do the fighting.